Hardening metal articles by nitrogenization



. that may be used are those of tin and lead and Patented Jan. 31, 1933 UNITED STATES PATENT OFFICE PIERRE FRANQOIS MARIE AUBERT, ADRIEN JULES PIERRE DUVAL, AND HENRI v AUGUSTE MARGUERITE DUVAL, 0F PARIS, FRANCE, ASSIGNORS, BY MESNE ASSIGN- MENTS, TO THE NITRALLOY CORPORATION, A CORPORATION OF DELAWARE HARDENING METAL ARTICLES BY NITROGENIZATION No Drawing. Application filed October 18, 1927, Serial No. 227,047, and in Germany May 10, 1927.

Renewed April 4, 1930.

Our invention relates to the known process of hardening metal articles by nitrogenization and has in particular reference to a method for preventing certain portions of the articles to be hardened in this manner from acquiring the nitrogenization hardness. Our method may be carried out in the following manner.

The metal articles to be hardened by nitroenization are covered, on the places where they are to be prevented from acquiring the nitrogenization hardness, with a coating of a metal or of an alloy, which during the hardening operation protects these places from the penetration of the nitrogen. It has been proved to be particularly suitable to apply on the places to be protected a coating of tin or of a tin alloy, this coating being applied either by the usual tinning process or by dipping the parts of the article to be protected a few minutes in a bath of liquid tin or a tin alloy. Examples of tin alloys of tin, lead and antimony. Lead-tin alloys containing up to 75% of lead may be used and have the advantage of cheapness. After removing from the bath the article thus partially tinned, the piece shows on the dipped parts a. coating of varying and sometimes considerable thickness depending upon the temperature and composition of the coating metal.

The metallic coating so applied will prevent nit-rogenization of the coated areas through several successive nitrogen hardening operations as tests have proved. This is true notwithstanding the fact that the ordinary nitrogenizing temperature is well above the fusion point of tin or the tin alloys used. l/Ve have not positively determined why the protective coating persists at temperatures above the-melting points of tin or the tin alloys used. While we do not intend to be bound by any theory advanced, this may be due either to the formation of a thin film of a tin-iron alloy, or merely to the for mation of an extremely thin layer of the tin or tin alloy which is 0t driven 01f at the nitrogenizing temperat ire or only after prolonged and repeated exposure to such temperature. When a tinned portion is to be hardened subsequently, the tin layer must be removed down to a certain depth.

As the melting temperature of the tin or tin alloy is lower than the temperatures ordinarily employed in the nitrogen hardenlng operation, any excess of tin ortin alloy over that necessary to form the thin protective film above mentioned melts during the nitrogenization and flows over the portions of the piece to be treated which are to be nitrogenized. To obviate the drawbacks resulting therefrom, we preferably cover the tinned places with a layer of a material that is capable of resisting the nitrogenizing temperature and thereby of preventing the tin layer from flowing over onto the portions to be nitrogenized. This layer may, for example, consist of a mixture of sodic water glass (sodium silicate) and powdered aluminium.

Another method for obviating the drawbacks mentioned may consist in dipping the tinned places or the whole piece in a salt bath heated to about 400 C., e. g., a bath of nitrate of sodium or nitrate of potassium. In the bath the excess tin becomes liquid and flows olf, without being able to adhere on the places which have not been tinned. The reason thereof resides in the fact, that the untinned places undergo a slight oxidation in the salt bath and that, besides, the tin drops cover themselves with an oxide layer, so that they are unable to exert a tinning effect. This slight oxidation of the untinned places does not prevent them in any way from acquiring the nitrogenization hardness. This method is being claimed in our copending application Serial No. 535,290, filed May 5, 1931.

After having been treated in the indicated manner, the pieces are subjected to the nitrogen hardening operation proper, in which the untinned places acquire the known nitrogen hardness, whilst the places previously tinned and either covered with a coating of water glass and aluminium powder or dipped in the salt bath remain so soft that they may be machined without difficulty.

The present invention is particularly applicable for use in conjunction with the nitrogenizing process described and claimed in U. S. patent to Fry, No. 1,487,554, although it is to be understood that it can be used in conjunction with other nitrogenizing processes.

What we claim and desire to secure by Letters Patent, is

1. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the port-ions to be protected a coating of tin or a tin alloy, and thereafter subjecting said article to a nitrogenizing treatment.

2. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the portions to be protected a coating of a metallic composition incapable of substantial penetration by nitrogen at a nitrogenizing temperature, and fusible at such temperature, applying on the portions so covered a layer of a material that is infusible at a nitrogenizing temperature, to retain said coating in place during the hardening process, and thereafter subjecting said article to a nitrogenizing treatment.

3. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the portions to be protected a coating of a metallic composition incapable of substantial penetration by nitrogen at a nitrogenizing temperature, and fusible at such temperature, applying on the portions covered with said coating 2. layer of a mixture of water glass and powdered aluminum, and thereaftersubjecting said article to a nitrogenizing treatment.

4. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the portions to be protected a coating of tin or a tin alloy,

applying on the portions covered with said coating a layer of a mixture of water glass and powdered aluminum, and thereafter subjecting said article to a nitrogenizing treatment.

5. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the portions to be protected a coating of a metallic composition incapable of substantial penetration by nitrogen at a nitrogenizing temperature and fusible at such temperature, treating the thus coated article to prevent spreading of said coating during the nitrogenizing operation to portions to be nitrogenized and thereafter subjecting said article to a nitrogenizing treatment. I i

6. In the hardening by nitrogenization of selected portions of a metal article, the steps which comprise applying on the portions to be protected a coating of tin or a tin alloy, treating the thus coated article to prevent spreading of said coating during the nitrogenizing operation toportions to be nitrogenized and thereafter subjecting said article to a nitrogenizing treatment.

The foregoing specification signed at Paris, France, this 3rd day of October, 1927.

PIERRE FRANCOIS MARIE AUBERT.

ADRIEN JULES PIERRE DUVAL. HENRI AUGUSTE MARGUERITE DUVAL. 

